1. Field of the Invention
The invention relates to an arrangement for measuring properties of a moving paper web, which arrangement comprises measuring means for measuring properties of a paper web and a supporting member for supporting the paper web in connection with the measuring means.
Sections where the paper web is not supported by a wire, for example, are called open draws. Open draws cause flutter in the paper web, and particularly because of an increase in the speed of paper machines, open draws easily lead to web breaks, which cause considerable costs. Thus, the intention is to support the paper web in the paper machine as much as possible, for example with wires. However, during the manufacture, web properties are measured while the web is moving, which means that frequently, if required by the measuring method, for example, the measurements have to be performed without a wire that would support the web. Thus, open draws have to be used to perform the measurement.
2. Description of Related Art
U.S. Pat. No. 4,194,840 discloses a measuring device comprising a convex lens arranged to be in contact with the paper web to be measured. U.S. Pat. No. 3,992,100, in turn, discloses a measuring device comprising a spherical surface in contact with the paper web to be measured, which surface encircles the measuring window. In both of the above solutions, the means in contact with the paper web leave scratches and traces on the paper web. Further, although contacting the paper web, the measuring means are not able to support the paper web sufficiently.
WO Publication 97/10383 discloses one solution for supporting the web during measurement. In this solution, sensing means measuring properties of a paper web are positioned in a carriage arranged to traverse back and forth in the cross-direction relative to the direction of travel of the paper web. Plane-like members, such as a flexible belt, are arranged in connection with the carriage to support the paper web. The belt is attached to both edges of the measuring head, a roll being arranged on both sides of the wire, whereby the belt is uncoiled from the roll and coiled onto the roll according to the reciprocating motion of the measuring head. Thus, there is no belt or any other element supporting the paper web at the measuring windows, but inside a measuring beam in the lateral direction of the carriage, the paper web is supported during the whole of the measurement. Moreover, the WO publication discloses a solution for supporting a paper web after it has detached from the wire or felt before the measuring beam and after the measuring beam before the wire. The publication suggests the use of air blow pipes extending over the whole width of the web and plane-like members attached to them, which form a plane-like surface supporting the web. Flutter of the paper web can be decreased and the speed of the paper web increased by means of these solutions, since no open draws are required for the measurements. However, the solution is mechanically rather demanding to implement. Further, the solution cannot be used in measurements in which the measuring device requires a view over the whole width of the paper web.